Knowledge on particle deposition in children's lungs is essential for realistic estimates of health risks posed by aerosol exposure of children, and also to determine the correct dose for therapeutic drugs delivered by inhalation, but this problem has received little attention. Currently, it is assumed that children's lungs are a small-size version of the adult one, and scaled-down adult lung models are used for the assessment of deposition efficiency. It is well established, however, that during postnatal development the lungs are not only growing in size, but they also undergo dramatic structural changes. Our previous studies provided evidence that the structure of the lung periphery is a major determinant of particle behavior; particularly, we have discovered a new mechanism of particle transport -chaotic mixing-, operating in the fully alveolated ducts of adult lungs. We hypothesize that age-associated changes in lung anatomy, such as progressive alveolation, lead to major changes in particle mixing and deposition, predicting that deposition peaks in very young children with lungs that have become largely alveolated already but are still small in size. To test this hypothesis, we propose to perform a study combining theoretical and experimental approaches building on our expertise. (1) We will perform analytical and numerical analyses to elucidate the relationship between progressive alveolation and the appearance of chaotic flow phenomena and associated mixing. (2) We will test the findings of these mathematical predictions in simplified and well-controlled physical models with expandable walls and different degrees of alveolation. (3) Excised lungs of developing piglets at different stages of alveolation will be used to do inhalation exposure and flow visualization experiments. The results of these approaches taken together will give us a comprehensive view of the relationship between alveolar geometry and the appearance of chaotic mixing mechanisms in the lung that may result in a sudden change in the extent of particle deposition. At the same time the findings of this project may lay down the foundation for realistic and rationally based inhalation dosimetry in children - a matter of great clinical importance. [unreadable] [unreadable]